NativeWindow.AssignHandle Method (IntPtr)


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Assigns a handle to this window.

Namespace:   System.Windows.Forms
Assembly:  System.Windows.Forms (in System.Windows.Forms.dll)

public void AssignHandle(
	IntPtr handle


Type: System.IntPtr

The handle to assign to this window.

Exception Condition

This window already has a handle.


The windows procedure for the associated native window could not be retrieved.

WndProc intercepts window messages sent to the handle parameter. Use ReleaseHandle to reset the handle's window procedure to the default window procedure.

The AssignHandle method calls the OnHandleChange method to indicate that the value of the Handle property has changed.


The handle to assign cannot be in a different application process.

The following code example demonstrates intercepting operating system window messages in a window procedure. The example creates a class that inherits from NativeWindow to accomplish this.

The MyNativeWindowListener class hooks into the window procedure of the form passed into the constructor, and overrides the WndProc method to intercepts the WM_ACTIVATEAPP window message. The class demonstrates the use of the AssignHandle and ReleaseHandle methods to identify which window handle the NativeWindow will use. The handle is assigned based upon the Control.HandleCreated and Control.HandleDestroyed events. When the WM_ACTIVATEAPP window message is received, the class calls the form1ApplicationActivated method.

This code is an excerpt from the example shown in the NativeWindow class overview. Some code is not shown for the purpose of brevity. See NativeWindow for the whole code listing.

// NativeWindow class to listen to operating system messages.
[System.Security.Permissions.PermissionSet(System.Security.Permissions.SecurityAction.Demand, Name = "FullTrust")]
internal class MyNativeWindowListener : NativeWindow

    // Constant value was found in the "windows.h" header file.
    private const int WM_ACTIVATEAPP = 0x001C;

    private Form1 parent;

    public MyNativeWindowListener(Form1 parent)

        parent.HandleCreated += new EventHandler(this.OnHandleCreated);
        parent.HandleDestroyed += new EventHandler(this.OnHandleDestroyed);
        this.parent = parent;

    // Listen for the control's window creation and then hook into it.
    internal void OnHandleCreated(object sender, EventArgs e)
        // Window is now created, assign handle to NativeWindow.
    internal void OnHandleDestroyed(object sender, EventArgs e)
        // Window was destroyed, release hook.
    [System.Security.Permissions.PermissionSet(System.Security.Permissions.SecurityAction.Demand, Name = "FullTrust")]
    protected override void WndProc(ref Message m)
        // Listen for operating system messages

        switch (m.Msg)
            case WM_ACTIVATEAPP:

                // Notify the form that this message was received.
                // Application is activated or deactivated, 
                // based upon the WParam parameter.
                parent.ApplicationActivated(((int)m.WParam != 0));

        base.WndProc(ref m);

.NET Framework
Available since 1.1
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